This is a randomized, double-blind, placebo-controlled study to evaluate the efficacy, safety and tolerability of recombinant-Methionyl Human Brain-Derived Neurotrophic Factor (r-metHuBDNF) when given by intrathecal infusion to subjects with amyotrophic lateral sclerosis. It will investigate the efficacy of the study substance on survival and freedom from permanent assisted ventilation. Each subject will receive an implanted pump which will provide either the study substance or a placebo. Upon completion of the study, subjects will have the option to receive open-label r-metHuBDNF for up to 12 months.